The jihadist Islamic State group took control of the rebel-held half of the eastern Syrian city of Deir Ezzor on Monday, buoyed by advances in neighbouring Iraq, a monitoring goup said.
Rival rebel groups fighting forces loyal to President Bashar al-Assad either gave in and joined the IS or fled from the strategic Euphrates valley city, the Syrian Observatory for Human Rights said.
Assad's forces remain in control of the other half of the city, a provincial capital and pre-war hub of the country's oil industry.
According to the Britain-based Observatory, which relies on a network of activists and medics on the ground, IS jihadists are now in control of "95 to 98 percent of Deir Ezzor province".
Only the regime-controlled half of Deir Ezzor city, a handful of villages and the military airport remain out of IS control.
The Observatory said that rivals of IS, including fighters of Al-Qaeda's Syria affiliate Al-Nusra Front, lost control after negotiations failed with the jihadists who last month declared a "caliphate" straddling Syria and Iraq.
"Fighters from Al-Nusra Front and the Islamist rebel movement Ahrar al-Sham withdrew from their bases in the city, while others pledged their loyalty to IS," the Observatory said.
"IS killed the Deir Ezzor chief of Al-Nusra, and raised their flag in the city," it added.
The rebel spokesman for Deir Ezzor confirmed the reports, blaming international backers of the anti-Assad opposition for a lack of support.
Speaking to AFP via the Internet, Omar Abu Leyla said: "The withdrawal is a result of the lack of any formal financial backing (for the rebels) either from the (exiled) opposition or from the international community."
IS seized huge supplies of weapons and money in Iraq's second city Mosul last month in a lightning offensive that saw it capture a swathe of territory north and west of Baghdad.
Its gains in Iraq have tipped the balance in the struggle for power in rebel-held areas of eastern and northern Syria where it has been fighting rebels and their Al-Nusra allies since January.
IS already controls the city of Raqa upstream from Deir Ezzor where it has earned a reputation for brutality in enforcing its radical form of Islam, with public executions, including crucifixions.
Abu Leyla added: "IS has no shortage of weapons, ammunition or fighters, and the battle became totally asymmetrical, especially after its advance on Mosul and its capture of heavy weapons."
He berated international opposition backers for failing to adequately arm Deir Ezzor's rebels and said: "We warned the world, including regional powers, of the dangers posed by this terrorist group (IS)."